Optical accelerometer

ABSTRACT

An accelerometer for providing digital output signals indicative of predetermined levels of acceleration in either direction along a sensitive axis therein. A mass is displaced a defined distance whenever a predetermined acceleration is experienced. This displacement aligns an optical fiber path contained by the mass between a light source and a light sensor causing the light sensor to produce an output signal. Different levels of acceleration can be detected by the use of a plurality of optical fiber paths within the mass.

Uited Sta. L,

m BfiBlpfEiQ-Z SEARCH ROOM 11 3,731,542 Forsberg SUBSTITUTE FOR M'TSSINGXR May 8, 1973 54] OPTICAL ACCELEROMETER 2,831,670 4/1958 Bournsetal...,... ....73/516 R l Inventor: John D g y Minn. 3,463,931 8/1969Kormos ..250/23l R [73] Assignee: Honeywell Inc., Minneapolis, Minn.Primary Examiner-James J. Gill [22] Filed: Man 26 1971 Attorney-CharlesJ. Ungemach and Albin Medved [21] Appl. No.1 128,260 [57] ABSTRACT Anaccelerometer for providing digital output signals U-S- Cl- R indicativeof predetermined levels of acceleration in [51] Int. Cl. ..G0lp 15/08either direction along a sensitive axis therein A mass [58] Field OfSearch ..73/5l4, 517 R, 515, is displaced a defined distance whenever apredeter, 73/516 517 71-3; 250/224 mined acceleration is experienced.This displacement 227; 116/114 AH; 340/66, 2 aligns an optical fiberpath contained by the mass between a light source and a light sensorcausing the [56] References C'ted light sensor to produce an outputsignal. Different UNITED STATES PATENTS levels of acceleration can bedetected by the use of a plurality of optical fiber paths within themass. 3,024,364 '3/[962 Wanetick...... ....250/83.3R 3,566,140 2/1971Granqvist 250/227 6 Claims, 5 Drawing Figures g 3x 23- c L f Tin. 2'

OPTICAL ACCELEROMETER BACKGROUND OF THE INVENTION The present inventionis an improvement in accelerometers.

Accelerometers which utilize and inertial mass and optical apparatus forthe detection of the displacement of that mass as a measurement ofacceleration exist in the prior art. However, the prior artaccelerometers just described are complicated in structure and are notsuitable for applications requiring high reliability under severevibration and acceleration conditions. Furthermore, they produce ananalogue output of the acceleration in only one direction along asensitive axis.

The present accelerometer has a simplified construction and operationallowing it to operate reliably under severe conditions. A digitaloutput is produced that indicates discrete positions of the inertialmass, thereby providing distinct output signals for a plurality ofpredetermined levels of acceleration in either direction along asensitive axis.

BRIEF SUMMARY OF THE INVENTION A light source and a plurality of lightsensors arelocated within the housing adjacent the mass. A plurality ofoptical fiber paths are contained within the mass at intervals along thelength of the mass. The space within the housing contains a dampingfluid. As the accelerometer experiences acceleration along the sensitiveaxis, the mass is displaced from its predisposed position. Each of theoptical fiber paths is located within the mass so that when apredetermined accelera tion occurs along the sensitive axis, an opticalfiber path is aligned between the light source and one of thelightsensors causing the light sensor to produce an out-.

put signal. Each optical fiber path is placed to cause an output signalfrom a light sensor for each predeter mined level of acceleration alongthe sensitive axis.

BRIEF DESCRIPTION OF THE-DRAWINGS FIG. I is a cross section view of apreferred embodiment of the accelerometer;

FIG. 2 is a cross section view of the embodiment of FIG. 1 taken alongthe line 2-2 of FIG. 1;

FIG. 3 is a cross section view of the preferred embodiment with noacceleration along the sensitive axis;

FIG. 4 is a cross section view of the preferred embodiment wherein thefirst level of acceleration along the sensitive axis is sensed; and IFIG. 5 is a cross section view of the preferred embodiment wherein asecond level of acceleration along the sensitive axis is sensed.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGS. I and 2, ahousing I0 has a cylindrical cavity 11 therein. A cylindrical mass 12 iscontained within cavity 11 and is free to move along a sensitive axis 13which is colinear with the axis of cylindrical mass 12. Housing 10 has aguide pin 14 protruding into cavity lll. Cylindrical mass 12 has alinear recess 15 along its surface aligned with sensitive axis 13.Recess 15 is shaped to accept guide pin 14 for maintaining apredetermined orientation about sensitive axis 13 between cylindricalmass 12 and housing 10.

An optical fiber path 16 is contained by housing 10, and is shaped totransmit two beams of light via a channel I7 and a channel 118 from alight source 19. The plane containing channels 17 and 118 isperpendicular to the axis of cylindrical mass 12 and contains a zeroacceleration axis 26 which is perpendicular to sensitive axis H3 andequidistant between the flat surfaces of cavity II. A pair ofphototransistors 20 and 21 are contained by housing It).lPhototransistors 20 and 21 produce an output signal when activated bylight. A pair of optical fiber paths 22 and 23 are located withinhousing I16) for directing light from cavity 11 to phototransistors 20and 21 respectively.

Cylindrical mass 12 contains four optical fiber paths 24, 25, 27, and28. Fiber paths 24 and 25 extend through mass 112 equidistant from andon opposite sides of axis 26, where axis 26 passes through mass 12equidistant from the flat surfaces of mass 12. Fiber paths 24 and 25 arepositioned in mass 12 to transmitlight from channel 117 to fiber path 22when mass 12 isdisplaced an equal distance from axis 26 in eitherdirection along sensitive axis 13. Fiber paths 24 and 25' havesufficient width parallel to sensitive axis 13 to maintain opticalcontact between channel 17 and fiber path 22 for greater displacementsof mass I12 due to increased acceleration, thereby indicating that theexisting acceleration continues to exceed a predetermined level. It maybe necessary that fiber paths 24 and 25 have a width that maintainsoptical contact between channel l7 and fiber path 22 to the maximumdisplacement of mass 12. Fiber paths 27 and 28 also extend through mass12 equidistant from and on opposite sides of axis 26 for transmittinglight from channel 18 to optical fiber path 23 when mass 12 is displaceda predetermined distance in either direction along sensitive axis 13.However, fiber paths 27 and 28 are further from axis 26 than are fiberpaths 24 and 25. The result is that optical contact between channel 18and fiber path 23 is established only when mass 12 has been displaced agreater distance than is necessary for optical contact between channelI7 and optical fiber path 22.

Mass 12 is maintained at an equilibrium position during the absence ofacceleration along sensitive axis 13 by a pair of calibrated loadingsprings 29 and 30 which are positioned within cavity III for compressionwhen acceleration is experienced from either direction along sensitiveaxis 113. When mass 12 is at its equilibrium position, zero accelerationaxis 26 is equidistant between fiber paths 24 and 25 and also betweenfiber paths 27 and 28. The remaining space within housing 10 is filledwith a damping fluid 31 which does not vary its viscosity substantiallyover a desired temperature range. Also, damping fluid 31 must notsubstantially refract or suppress light transmission.

OPERATION The accelerometer described herein provides a digital outputindicative of predetermined levels of acceleration in either directionalong a sensitive axis. In

the absence of acceleration along the sensitive axis, cylindrical mass12 remains at its equilibrium position shown in FIG. 3 wherein mass 12is centered about zero acceleration axis 26. In that position, no outputsignal is produced since mass 12 prevents light emitted by light source19 from reaching phototransistors 20 and 21. As acceleration appearsalong sensitive axis 13, mass 12 compresses either load spring 29 orload spring 30 depending upon which direction along sensitive axis 13the acceleration is directed. Assuming that an acceleration forceappears along sensitive axis 13 from the left of axis 26 as viewed inH0. 4, load spring 29 is compressed between mass 12 and housing due tothe inertial reaction of mass 12. However, no output signal is produceduntil mass 12 has been displaced to the position shown in PK}. 4. Inthat position optical fiber path 25 is aligned between channel 17 ofoptical fiber path 16 and optical fiber path 22 thus completing anoptical circuit between light source 19 and phototransistor 20. Uponreceiving a beam of light via this optical circuit, the phototransistorproduces an output signal thereby indicating that the predeterminedlevel of acceleration for which optical fiber path is located to measurehas occurred.

Referring to FIG. 5, as the acceleration along sensitive axis 13increases its magnitude from the left of axis 26, load spring 29 isfurther compressed by mass 12 until optical fiber path 28 is alignedbetween channel 18 of fiber path 16 and opticle fiber path 23 therebycompleting an optical circuit between light source 19 andphototransistor 21. That circuit allows light from light source 19 toactivate phototransistor 21 causing a second output signal indicative ofa second level of acceleration along sensitive axis 13.

Two distinct levels of acceleration occurring along sensitive axis 13from the right of zero acceleration axis 26 are similarly detected. Asmass 12 is displaced to the right of axis 26 by an acceleration forcefrom the right of axis 26, load spring is compressed until optical fiber24 is aligned between channel 17 and optical fiber path 22. An opticalcircuit is then completed which allows light source 19 to activatephototransistor 20. Increased acceleration from the right brings opticalfiber path 27 into alignment between channel 18 and optical fiber path23, thereby completing a second optical circuit that causesphototransistor 21 to produce an output signal.

Additional levels of acceleration could be detected by placingadditional optical fiber paths within inertial mass 12 for completingoptical circuits between additional light sources and phototransistors.

The orientation of light source 19 and phototransistors 20 and 21 aboutinertial mass 12 is not critical but need only be oriented such that anoptical circuit is completed via an optical fiber path in response tothe movement of mass 12. A discrete level of acceleration is indicatedwhen the movement of mass 12 aligns the edge of an optical fiber paththerein with the edges of the optical fiber paths in housing 10 thatlead to the light sources and the phototransistors.

It may be desired to eliminate the optical fiber paths within housing111 by placing phototransistors 20 and 21 and light source 19 adjacentcavity 11 with their lightconductingedgespositioned ataxis 26.

lhe embodiment of the present invention ust described is preferred.Variations in the preferred embodiment may be apparent to one ofordinary skill in the art. Such variations will not alter the scope andprinciple of the present invention.

I claim as my invention:

1. An accelerometer for indicating distinct bidirectional levels ofacceleration along a sensitive axis comprising:

a housing;

an inertial mass mounted within said housing for motion along saidsensitive axis;

biasing means within said housing for maintaining said mass in apredetermined position along said sensitive axis in response topredetermined acceleration;

a source of light at a first location within said housing;

a light detector at a second location within said hous ing normallyblocked from said source by said inertial mass; and

light transmitting means within said inertial mass for transmission oflight from said source to said de tector only when said inertial mass isdisplaced a predetermined distance along said sensitive axis in responseto an acceleration force of a predeter mined magnitude.

2. The accelerometer of claim 1 wherein said light transmitting means isan optical tibgr path.

3. The accelerometer of claim 1 wherein said light transmitting means isa plurality of optical fiber paths, each of said fiber paths beingpositioned within said mass for transmitting light from said source to aplurality of light detectors due to a different predetermined level ofacceleration. I

4. The accelerometer of claim 3 wherein said biasing means comprises apair of calibrated load springs.

5. The accelerometer of claim 4 and a damping fluid contained withinsaid cavity.

6. The accelerometer of claim 5 and a means attached within said housingfor maintaining said mass in a predetermined orientation about saidsensitive axis.

1. An accelerometer for indicating distinct bidirectional levels ofacceleration along a sensitive axis comprising: a housing; an inertialmass mounted within said housing for motion along said sensitive axis;biasing means within said housing for maintaining said mass in apredetermined position along said sensitive axis in response topredetermined acceleration; a source of light at a first location withinsaid housing; a light detector at a second location within said housingnormally blocked from said source by said inertial mass; and lighttransmitting means within said inertial mass for transmission of lightfrom said source to said detector only when said inertial mass isdisplaced a predetermined distance along said sensitive axis in responseto an acceleration force of a predetermined magnitude.
 2. Theaccelerometer oF claim 1 wherein said light transmitting means is anoptical fiber path.
 3. The accelerometer of claim 1 wherein said lighttransmitting means is a plurality of optical fiber paths, each of saidfiber paths being positioned within said mass for transmitting lightfrom said source to a plurality of light detectors due to a differentpredetermined level of acceleration.
 4. The accelerometer of claim 3wherein said biasing means comprises a pair of calibrated load springs.5. The accelerometer of claim 4 and a damping fluid contained withinsaid cavity.
 6. The accelerometer of claim 5 and a means attached withinsaid housing for maintaining said mass in a predetermined orientationabout said sensitive axis.